The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Emissions control is a factor in engine design and engine control. NOx, a known by-product of combustion, is created by nitrogen and oxygen molecules present in engine intake air disassociating in the high temperatures of combustion. Rates of NOx creation follow known relationships to the combustion process, for example, with higher rates of NOx creation being associated with higher combustion temperatures and longer exposure of air molecules to the higher temperatures. Reduction of NOx created in the combustion process and management of NOx in an exhaust aftertreatment system are desirable in vehicle design.
NOx molecules, once created in the combustion chamber, can be converted back into nitrogen and oxygen molecules in exemplary devices known in the art within the broader category of aftertreatment devices. However, one having ordinary skill in the art will appreciate that aftertreatment devices are largely dependent upon operating conditions, such as device operating temperature driven by exhaust gas flow temperatures. Additionally, aftertreatment devices include materials, such as catalyst beds, prone to damage or degradation as a result of use over time and exposure to high temperatures.
Modern engine control methods utilize diverse operating strategies to optimize combustion. Some operating strategies, optimizing combustion in terms of fuel efficiency, include lean, localized, or stratified combustion within the combustion chamber in order to reduce the fuel charge necessary to achieve the work output required of the cylinder. While temperatures in the combustion chamber can get high enough in pockets of combustion to create significant quantities of NOx, the overall energy output of the combustion chamber, in particular, the heat energy expelled from the engine through the exhaust gas flow, can be greatly reduced from normal values. Such conditions can be challenging to exhaust aftertreatment strategies, since, as aforementioned, aftertreatment devices frequently require an elevated operating temperature, driven by the exhaust gas flow temperature, to operate adequately to treat NOx emissions.
Aftertreatment devices are known, for instance, utilizing chemical reactions to treat constituents in the exhaust gas flow. One exemplary device includes a selective catalytic reduction device (‘SCR’). An SCR utilizes a reductant capable of reacting with NOx to treat the NOx. One exemplary reductant is ammonia derived from urea injection or recovered through catalytic reaction of components of the exhaust gas flow. This disclosure will discuss ammonia generically as a reductant, however, it will be appreciated that a number of reductants are known in the art and are contemplated in this disclosure. Ammonia stored on a catalyst bed within the SCR reacts with NOx, preferably NO2, and produces favorable reactions to treat the NOx. It is known to operate a diesel oxidation catalyst (‘DOC’) upstream of the SCR in diesel applications to convert NO into NO2 preferable to treatment in the SCR. Continued improvement in exhaust aftertreatment requires accurate information regarding NOx emissions in the exhaust gas flow in order to achieve effective NOx reduction, such as dosing proper amount of urea based on monitored NOx emissions.
Other aftertreatment devices are additionally known for treating constituents in the exhaust gas flow. Three way catalysts (‘TWC’) are utilized particularly in gasoline application to treat constituents. Lean NOx traps (‘NOx trap’) utilize catalysts capable of storing some amount of NOx, and engine control technologies have been developed to combine these NOx traps or NOx adsorbers with fuel efficient engine control strategies to improve fuel efficiency and still achieve acceptable levels of NOx emissions. One exemplary strategy includes using a lean NOx trap to store NOx emissions during fuel lean operations and then purging the stored NOx during fuel rich, higher temperature engine operating conditions with conventional three-way catalysis to nitrogen and water. Diesel particulate filters (‘DPF’) trap soot and particulate matter in diesel applications, and the trapped material is periodically purged in high temperature regeneration events.
Aftertreatment devices such as SCR devices convert NOx to other molecules at some conversion efficiency. NOx conversion efficiency can be described by the flow of NOx flowing into a device versus the flow of NOx exiting the device. Reduced conversion efficiency within an SCR device can result from a number of conditions. Malfunctions or degraded performance caused by wear or damage can reduce the efficiency of the aftertreatment device. Additionally, an SCR device otherwise operating properly can experience reduced efficiency according to properties of the exhaust gas flow that affect the chemical reaction occurring in the device. For example, temperature and space velocity of the gases within a NOx trap affect the efficiency of the device. Temperature and space velocity of the gases within an SCR device similarly affect the efficiency of the device. These environmental factors can be monitored in the aftertreatment system, and effects of these factors upon device conversion efficiency can be estimated. Additionally, failure of the system to replenish ammonia within the SCR device results in reduced efficiency, and elevated levels of ammonia slip or dissipation result in variability in efficiency. Tests to evaluate malfunction catalysts are known, for example by evaluating a current conversion efficiency versus an expected conversion efficiency. However, false indications of a malfunction catalyst are possible depending upon whether reduced conversion efficiency is a result of a malfunctioning device or conditions within the device. A method to distinguish degraded performance based upon transient environmental conditions from a malfunctioning or damaged aftertreatment device, for example, in the form of conditions required to initiate a test for a malfunction catalyst, would be beneficial to diagnosing a malfunction condition in the device.